1. Field of the Invention
This invention relates to inverters for motor drives and more specifically, relates to inverters that include III-nitride based power semiconductor devices.
2. Description of the Art
Referring to FIG. 1, there is shown a schematic of a motor drive 102 for a three phase AC electric motor 130 according to the prior art. Motor 130 may be an induction motor or a brushless DC (BLDC) motor, for example. Motor 130 includes a stator having a three winding system A, B, and C and further includes a rotor having a shaft, the rotor/shaft being disposed within the stator (note that FIG. 1 does not show the stator, rotor, and shaft). Motor drive 102 includes a controller 104 and an inverter 110. Inverter 110 is a three phase inverter that includes three power stages 118, 119, and 120 that are connected in parallel across a DC voltage bus 106 and ground lead 108. Each power stage is connected to a respective one of the three windings A, B, and C of motor 130. Controller 104 is connected to each of the power stages through six control leads, labeled 105a-f, and provides on these leads control signals that configure the three power stages to generate power signals that drive motor 130. In particular, through the control signals provided by controller 104, power stages 118-120 of inverter 110 produce three, phase-displaced AC power signals that energize windings A, B, and C of motor 130, thereby generating a rotating magnetic field that excites the motor rotor and thereby spins the motor shaft.
Referring more specifically to inverter 110, each power stage 118-120 of the inverter includes a high side switch and a low switch, labeled as 112a-f, that are connected in series across DC voltage bus 106 and ground lead 108. Each winding A, B, and C of motor 130 is connected at the junction of a respective high side and low side switch of each stage.
As is known, the current through motor windings A, B, and C changes direction during the motor operation. Accordingly, each high side and low side switch 112a-f of each power stage 118-120 is configured to include two power devices, including a unidirectional switch 114a-f, such as an IGBT, and a diode 116a-f that is short-circuited across the source and drain of the unidirectional switch. Unidirectional switches 114a-f handle the forward current through switches 112a-f and diodes 116a-f handle the reverse current through switches 112a-f. 
As shown in FIG. 1, the gate of each unidirectional switch 114a-f is connected to a respective control lead 105a-f of controller 104. In general, controller 104 generates control signals on the control leads that turn “on” and “off” the high side and low side switches 114a-f of the power stages, effectively transitioning the inverter through various configurations and thereby causing the inverter to produce the three, phase-displaced AC power signals that energize motor windings A, B, and C of motor 130.
Inverter 110 has several limitations as a result of diodes 116. In particular, the diodes have a larger voltage drop than switches 114 and as such, have a higher power dissipation than the switches. In addition, as controller 104 transitions the inverter through the different configurations and causes the diodes to transition from a conducting to a non-conducting state (i.e., “on” to “off”), the diodes discharge a reverse recovery charge (Qπ) that causes both a power loss in the switches and a radiated and conducted EMI noise.
For example, assume controller 104 has configured inverter 110 as shown in FIG. 2. Here, high side switch 114a and low side switches 114d and 114f are “on”. In addition to an energized current that flows through windings A and B, this configuration allows an inductive current to flow through winding C to winding B, down through switch 114d of stage 119 to ground lead 108, up through diode 116f of stage 120, and back to winding C. Accordingly, in this configuration, diode 116f is effectively “on” and operates as a flywheel diode that keeps the current in the motor windings flowing in the correct direction.
Assume next that controller 104 transitions inverter 110 to a configuration where low side switch 114f is “on” such that a current now flows through the switch to ground lead 108. In this configuration, diode 116f is effectively forced “off” by switch 114f. Notably, as diode 116f is turned “off”, a Qπ charge stored in the diode is discharged and must be removed. The removing of this charge is performed by switch 114f, and results in a turn-on switching/power loss in the switch. Notably, similar switching losses occur in the other power stages of inverter 110 as the diodes are forced “off”. These switching losses caused by the flywheel diodes of inverter 110 effect the thermal performance of the inverter, reduce the power density of the inverter, and increase cost. In addition, the removal of the stored Qπ charge by the switches also increases radiated and conducted EMI noise in the drive.
Notably, the switching losses and EMI noise caused by the diodes may be reduced by using diodes with a fast reverse recovery time (i.e., a small tπ value). However, reducing the reverse recovery time of the diodes increases other radiated and conducted EMI noise in inverter 110. For example, assume controller 104 has configured inverter 110 such that switches 114e and 114f of stage 120 are “of” and “on” respectively, and current is flowing up through diode 116f. Assume next that controller 104 transitions inverter 110 to a configuration where switches 114e and 114f of stage 120 are now “on” and “off” respectively, and diode 116f is forced “off”. As this occurs, the junction of switches 114e and 114f (i.e., the connection point of winding C) swings from 0 volts up to the voltage on DC voltage bus 106. This swing generates a sharp dV/dt as well as a fast di/dt, each of which are additional sources of radiated and conducted EMI noise. Again, similar radiated and conducted EMI noise will occur in the other power stages of inverter 110 as controller 104 transitions the inverter through the various configurations. Notably, the dV/dt may be limited by configuring the gate resistance (Rg) of the switches 114 and the di/dt may be controlled by configuring the diodes 116 to have a larger tπ value. However, as indicated above, a larger tπ value for the diodes increases power loss in the switches.
Accordingly, it is desirable to provide an inverter for a motor drive wherein the inverter in not limited by the adverse effects of the diodes.